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I wonder if the buffer tank should move to the retrun side of the chiller, and the chiller draws from the top and loop return is to the top, but the bypass is to the bottom of the tank. That way you get the warmest water to the chiller most of the time even when below minimum load.

I assume to lengthen minimum cycle times you might run the chiller to start at 44-45F and shut off at 40F.

I see what your saying, but depending on how large the tank is it might take a while before the chiller can catch up, and before you could have a decent enough leaving water temp. With the way I have it drawn out the FCU(s) can have presidence.

If I was able to actually build a system like the way I wanted it, I would want the tanks to be very large (3,000+ gallons total). I have free electricity after 10 pm and I would like to have thermal storage if possible. In the hottest of summers my unit runs 14 hours a day (3 ton unit). So I would need 42 ton/hrs. storage plus be able to recharge the tanks. Just shootin' in the wind I would guess that a 10 ton chiller like this that CraziFuzzy had mentioned earlier would work.

Yea, I know I'm dreaming...

"It's not that I'm smart, it's that I stay with the problem longer”
Albert Einstein

I've actually figured out that I would only need about 2 tons if running at night, to provide the cooling i need during the day, with about 2500 gallons of storage, but I wouldn't be able to use an off-the-shelf fan coil. I need one with a much larger chilled water coil, to provide full approach to return air temp to maximize the storage capacity. I think that's the primary problem with using that variable primary type system you posted - It is harder to ensure that max delta-t with that setup, and no way to always provide a fixed supply temp, regardless of return temp. You have to have that primary pump and 3-way recirc valve around the chiller, to ensure the outlet temp going to the storage tank is always at your supply setpoint. Running warm water into the cold end of a stratified tank is bad, as it destroys the thermocline, and overall degrades your recovery capability.

Is that 2500 gallons with or without the A/C also running during the day.

For my example, that was enough to keep the chiller OFF during the peak demand period (8 hours a day - 10am to 6pm). Power costs are the same for the other 16 hours of the day, so that was enough. That system i drew up a couple pages ago, that ran the chiller based on storage tank temps, would pretty much start topping off the tank as soon as the peak period ended.

That calc was also based on a chilled water supply temps of 44°F, and a full return temp of 74°F, which is why it would require a large multi-stage coil, but the higher the delta-t, the better stratification you get, and the more storage you can have per gallon.

Makes me wonder if you read an article in "supply house times" talking about small scale chilled after cooling as a developing market. The article was by John Siegenthaler, P.E..

I have not, but I'm not surprised to see the article. It IS a natural progression, and a single change that opens up much more opportunities with little extra engineering. Water is the perfect medium for heat transfer in homes, mainly because of it's safety, and, if for no other reason, is already piped to every home. It takes nothing to add on 'extra features' to an established chilled water system. Want to stretch out cycle times? add in some buffering storage. Want to shift loads (something that is a much harder proposition with direct cooling), just add a larger tank. Want to zone? sure, that's simple. Want to expand the system to that new addition? sure, just add another fan-coil there. Did the chiller just die? okay, go to the store and buy another one off the shelf - they are self contained units, not requiring any recovery, charging, vacuum, etc. I know THAT part the residential part of the industry doesn't want, but in the end, it's going to happen.

I was just trying to run some numbers of my own and I was wondering how you can up with your 2,500 mark. The last thing I have run across is this from ASHRAE.

All in all I'm coming up with some pretty broad numbers and I was wanting to nail something down.

That's a good question. I assume you'd have to know your average heat gain during design conditions. Then you determine what the 24 hour average is and then size your storage so your chiller is sized for hte average load and run continously but you have enough water storage.

Its' just thermal mass. You can accomplish a similar effect if you had enough concrete or masonry walls in a home. This is why IMO, energy efficient structure design needs ot encorporate materials when possible with a lot of thermal mass. SO using ICF's. I've mentioned in another thread, that I think you could easily raise the sill plate up to at least a point level with the bottom of the 1st floor windows. Homes should be 2 story and use stucco, brick or stone on the exterior. Have deep overhangs for shading the building in the summer, and less in the winter, use interior walls (non of this open concept garbage) and use stone, brick or plaster, instead of gypsum for interior walls too. Close cell spray foam of course and unvented attics with cool roofs.

Actually, with enough thermal mass in the structure, the need for water storage could then be sized to target a minimum cycle time at low load conditions.

I was just trying to run some numbers of my own and I was wondering how you can up with your 2,500 mark. The last thing I have run across is this from ASHRAE.

All in all I'm coming up with some pretty broad numbers and I was wanting to nail something down.

my 2500 gallon number is considerably oversized, but that is for a couple reasons. part of the space has to be taken up by the termocline layer, so not the entire volume of the storage is 'usable'. also, I figured bigger is better, as tanks really aren't THAT expensive in this size range (I was looking at splitting the storage among say 3 separate tanks plumbed in series). My actual 'conservative calc' was based on how much it would take to store 24 hours of the 2 ton chiller's capacity. As such, that's:

Previously, I had determined how much it would take to provide absolute worst case cooling on my home for the entire peak power period, which is about 3.5 tons for 8 hours. As such:

42,000 BTU/hr * 8 hr * 1 lb/BTU°F ÷ 8.34 lb/gal ÷ 30°F = 1342 gallons

so if all you are looking to do is shift load from the peak power period (for Time-of-use metering), it doesn't take THAT much of a storage system. It's also something that can easily be expanded upon. Starting with just 500 gallons will minimize the cycling, but adding tanks can seriously start to shift load.

I know prices can't be posted in the open forum, but I was wondering if anyone has any idea of the cost of the 10 ton multiaqua chiller is? I talked to multiaqua a couple of weeks ago just for them to tell me that Trane is the distributor for their equipment in the Dallas/Fort Worth area. I have called and e-mailed every contact in the D/FW Trane office and still have not heard anything. Good 'ol Trane. Their reputation will never change!!!!

"It's not that I'm smart, it's that I stay with the problem longer”
Albert Einstein